The invention relates to a projection television display tube comprising an evacuated envelope having a display window which on its inside has a display screen and in front of which on its outside a transparent second window is provided, a cooling liquid which flows through a space between the display window and the second window from at least one inlet aperture to at least one outlet aperture, said space communicating with an expansion vessel.
Such a display tube is disclosed in German Patent Application 3021431 laid open to public inspection to which U.S. Pat. No. 4,529,905 corresponds. A raster is scanned on the display screen, which usually consists of a layer of phosphor or a pattern of different phsphors, by means of an electron beam. Due to the electron bombardment, the temperature of the phosphor increases as a result of which the luminous efficiency of the display screen decreases (thermal quenching). This phenomenon occurs in particular in display tubes for projection television in which the display screen is scanned by electron beams with comparatively high beam currents to obtain the required high brightnesses. The temperature of the display window also increases and a temperature gradient is formed in the glass display window. With a high electron beam current and hence a high thermal load this may lead to fracture of the display window. In order to reduce these mechanical stresses in the display window as a result of temperature differences (thermal stress) and to mitigate the reduction of the luminous efficiency, it is known from the already mentioned published German Patent Application 30 21 431 to cool the display window and the display screen connected thereto. In a first described embodiment, a metal cooling member operating as a heat radiator is provided around the first and the second window and the space therebetween filled with coolant. As a result of the rise in temperature of the display window, the cooling liquid heated by the display window flows along the display window upwards and along the second window downwards, as a result of which the thermal energy of the centre of the display window is also dissipated via the cooling member. With a low load, for example less than 5 W, the thermal energy is dissipated mainly via conduction to the second window. At high load the above-described flow of liquid begins to occur with the associated, not very effective, extra cooling by the cooling member. Moreover an embodiment is described in which the cooling liquid is subjected to cooling outside the space. For that purpose the cooling liquid is conveyed by means of flow as a result of temperature differences in the cooling liquid from the top of the space via a tube connected to an outlet aperture to a cooling chamber and, via a second tube, from the cooling chamber to the inlet aperture on the lower side of the space. An embodiment is also described in which an expansion vessel communicates with the space between the first and the second window. This is in the form of bellows which follow the thermal expansion and shrinkage of the cooling liquid. A disadvantage of this bellows construction is that it is complicated and hence expensive.
It is therefore an object of the present invention to provide a simpler and cheaper construction.